1. Field of the Invention
The present invention relates generally to surgical knife blades and in particular to blades used in ophthalmological surgical procedures such as clear corneal incisions.
2. Description of the Prior Art
Ophthalmic surgeons work within a very small operating field upon organs whose tissues are complex and delicate. Cuts made during surgery must be precise as to length, direction and depth, requiring surgical knives of unsurpassed sharpness and maneuverability and with configurations particularly suited to operating upon selected portions of the eye.
It has become well-known to design blades for ophthalmological knives to perform specific cuts used in certain types of eye surgery. One type of ocular surgery used to correct cataracts is referred to as clear corneal cataract surgery, discussed in an article entitled "Beveled blades have simplified clear corneal technique" by William F. Maloney, M.D., appearing in the Sep. 15, 1997 issue of Ocular Surgery News in which clear corneal surgery is generally described as the use of a beveled diamond blade to make a three-step incision to form a path through the cornea and into the anterior chamber of the eye.
As mentioned by Dr. Maloney, examples of known beveled blades are the Rhein 3-D trapezoid blade manufactured by Rhein Medical, Inc. of Tampa Fla., the Accutome beveled diamond manufactured by Accutome of Malvern, Pa., the Storz multi-beveled diamond manufactured by Storz of St. Louis, Mo., and the pyramid diamond manufactured by KMI of Paoli, Pa. All feature a blade profile having a posterior surface formed generally as an elongated "home plate" type of pentagon with a leading, sharply V-shaped pointed tip, an anterior surface shaped generally to correspond to the posterior surface and a series of bevels extending from the anterior surface to the posterior surface. Cutting edges are formed where the two bevels intersect, or where a bevel intersects the anterior or posterior surface, most typically resulting in a pair of cutting edges diverging from a leading point or distal end of the blade along the legs of a V to intersect with a pair of lateral cutting edges formed along that portion of the blade extending from the distal end rearward toward the proximal end which is adapted to be gripped by a knife or blade holder. Certain known blade configurations feature lateral cutting edges that are substantially parallel while other configurations feature lateral cutting edges that diverge along the distal-to-proximal direction, allowing the width of the incision to be determined by the distance to which the blade is inserted through the corneal membranes, with the incision being widened as the blade is inserted.
Removing a damaged or diseased lens and replacing it with an artificial intraocular lens calls for the surgeon to make incisions in the cornea or the sclera through which fragments of the old lens are removed and through which the new lens is inserted. Techniques are now used to fold the new lens prior to insertion and to allow it to unfold once it is in place, requiring a relatively small, straight incision which heals quickly and, if properly made, limits or eliminates fluid leakage from the eye without requiring suturing or hydration.
The cornea is made up of several tissue layers through which an incision must be made to reach the anterior chamber of the eye, principally the anterior epithelium, Bowman's membrane and Descemet's membrane. A description of the problems inherent in making such incisions and the techniques presently required is found in U.S. Pat. No. 5,713,915 (Van Heugten, et al.). According to Van Heugten, et al., because the cornea is spherical in shape, such cuts tend to produce non-linear incision lines unless they are made at a 90.degree. angle to the surface being cut. One presently known technique for making linear, water-tight incisions is described as "dimpling down" which requires a surgeon to attempt to flatten the cornea, or "dimple down" as soon as the tip of the surgical blade reaches Descemet's membrane in order to create a substantially linear, perpendicular incision through the membrane. According to Van Heugten, et al., dimpling down requires the surgeon to lift the back of the blade up to point the tip of the blade down which can cause distortion in the tunnel formed by the cut. Lifting the back of the blade also increases the angle of the cut, making it less tangential to the circumferential arc of the cornea, affecting the water-tight integrity of the unsutured incision. To compensate, the surgeon must hydrate the corners of the incision.
U.S. Pat. No. 5,713,915 (Van Heugten, et al.) teaches and describes a surgical knife blade for use in ophthalmological surgery characterized by sharply pointed tip and a non-symmetrical lateral profile, with cutting edges formed by the intersection of differently-sized anterior and posterior bevels to position the cutting edges closer to the anterior surface than the posterior surface. Van Heugten, et al. state that curved incisions result most directly from the use of blades that are typically symmetrical when the anterior surface is compared to the posterior surface. Also claimed in Van Heugten, et al. are side edges formed by anterior and posterior bevels, with the side edges meeting the cutting edges at a shoulder and with the side edges and the cutting edges being equal at the shoulder.
Other prior art blades used to incise the cornea characteristically have sharply pointed tips and fall generally into two categories: symmetrical blades with cutting edges formed by bevels and positioned substantially midway between anterior and posterior blade surfaces, and blades with edges formed by a bevel formed on one blade surface intersecting the plane of the other surface.
U.S. Pat. No. 4,688,570 (Kramer, et al.) teaches and describes an ophthalmological surgical instrument used to guide a knife in cutting radial keratomatic incisions in the cornea. The knives shown in Kramer, et al. are of the type having anterior and posterior bevels intersecting midway through the blade's thickness to form the cutting edges.
U.S. Pat. No. 5,201,747 (Mastel) teaches and describes an ophthalmological surgical instrument having a triple edge tip using opposed bevels to form a symmetrical blade.
U.S. Pat. No. 5,217,476 (Wishinsky) teaches and describes a surgical knife blade and method of performing cataract surgery utilizing a surgical knife blade which is symmetrical and beveled to form a centrally-positioned cutting edge.
U.S. Pat. No. 5,224,950 (Prywes) teaches and describes a color calibrated multifunction scalpel blade for intraocular and other surgery and associated methods of use showing symmetrical cutting edges and a color-coded blade to indicate how deeply the blade has been inserted.
U.S. Pat. No. 5,376,099 (Ellis, et al.) teaches and describes an undercut diamond surgical blade and method of using the same having a non-symmetrical pointed cutting tip, the cutting edges of which are centered between the two surfaces of the blade and are formed by bevels on the blade sides.
U.S. Pat. Nos. 5,203,865 and 5,098,438 (Siepser) teach and describe surgical knives for use in ophthalmic surgery and procedures for intraocular surgery in which a variety of surgical knives are described of the type having parallel surfaces and cutting edges formed by the intersection of bevels extending from one face to the other.
U.S. Pat. No. 5,370,652 (Kellan) teaches and describes a surgical knife blade for making sutureless incisions in the eye and methods therefor which discloses several blade configurations with cutting edges formed by the intersection of a posterior bevel with the anterior surface of the blade.
U.S. Pat. No. 5,405,355 (Peyman, et al.) teaches and describes a method of radial keratotomy employing a vibrating cutting blade in which a triangular blade with cutting edges formed by blade face bevels is disclosed.
U.S. Pat. No. 5,222,967 (Casebeer) teaches and describes a keratorefractive diamond blade and surgical method illustrating a blade with a cutting edge formed by intersecting blade bevels.
U.S. Pat. No. 5,336,235 (Myers) teaches and describes a keratome having a curved, pointed blade with a cutting edge formed by the intersection of a bevel on the upper, curved surface with the lower, curved surface.
These references generally exemplify surgical blades having "pointy" blade tips, that is, blades whose cutting edges meet at an acute angle. It is believed that this design creates problems when making the type of incision required for clear corneal surgery. In particular, the available surface area of the blade available to support the tissue during cutting is limited when compared to the surface area of a blade whose lead cutting edges meet at a larger angle. Adopting the cutting profile of the present invention thus teaches away from the present art because such a profile would seem to require significantly more force to start a cut, a factor that must be balanced against the advantage of having a larger surface area to support the tissues being incised. However, this has not been the case. Blades made in accordance with the teachings of the present invention have been successfully used in clear corneal surgery.
It is an object of the clear corneal surgical technique to make an incision that seals itself and does not require sutures to prevent leakage of fluid from the anterior chamber of the eye. It has been found that a single lateral incision which extends partially through the cornea and then changes to a direction more approximating a line perpendicular to Descemet's membrane creates a path or tunnel through the corneal tissue which effectively seals itself and does not leak. The first segment of this incision through the outer corneal tissue is identified as the inner corneal valve, while the second, stepped portion of the incision is called the anterior chamber entry.
It is believed that one of the keys to making such an incision self-sealing is to maximize the surface area of the cut, that is, the surface area of the "roof" and "floor" of the tunnel. Another factor is the ability to keep the incision linear, that is, to keep the edges of the incision straight, not allowing them to tear or sag down. Another problem is thought to result when the blade is withdrawn after it has pierced Descemet's membrane. There are times when the edges of the incision are torn, presumably when the edges snag on a portion of the blade as it is withdrawn or when the surgeon is required to abruptly change the angle of the blade to dimple down. Such tears cause leaks.
It is also desirable to have a blade configuration which allows the surgeon to make accurate, reproducibly self-sealing incisions without having to estimate the depth of cut and without relying upon the need to change hand positions during the incision or to estimate the angle to which the blade must be brought to effectively dimple down and complete the incision.
Accordingly, it is an object of the present invention to provide a blade to be used for ophthalmological surgical procedures that will create a reproducibly self-sealing incision when used to penetrate the corneal tissue, eliminating the need for sutures.
It is also an object of the present invention to provide such blades in configurations which maximize the surface area of the incision to enhance the self-sealing action.
It is a further object to provide such blades in configurations which support the incision as it is being made to limit the tendency of the incision edges to sag or tear.
Another object is to provide such blades in configurations which automatically create a dimpling down action at a reproducible depth of cut through the corneal tissue without requiring the surgeon to change hand positions to select an entry angle.
Still another object of the present invention is to provide such blades in configurations which reduce the tendency of the blade to snag on the edges of the incision when the blade is being withdrawn.
It is clear from the foregoing that there is a demonstrated need for a surgical blade for use in ophthalmological surgical procedures which is capable of reproducibly creating a leak-proof corneal incision through which other surgical procedures may be carried out without requiring the surgeon to make adjustments in hand position or to determine visually when or to what extent the angle of cut should be changed.